This chapter presents an overview of the key concepts discussed in the subsequent chapters. The European Union (EU) has slowly grown from a geographically delimited Western European club of six members based on a rather narrow remit of action to an almost continent-wide union of 28 member states whose realm of action is wide-encompassing, indeed. A general assumption is that the EU is an international organisation made up of states whose main purpose is to serve the interests and concerns of the member states. The book focuses on the former group of states that qualify for EU membership, and entered into binding cooperation with the EU through different forms of agreements or through a distinct category of associated non-membership. A distinguishing and distinctive feature is that EU law enjoys direct effect and supremacy in those areas where the EU has been granted competence from the member states.